8.8 Network Assisted Cell Change procedures

3GPP44.060General Packet Radio Service (GPRS)Mobile Station (MS) - Base Station System (BSS) interfaceRadio Link Control / Medium Access Control (RLC/MAC) protocolRelease 17TS

8.8.1 Neighbour Cell System Information Distribution

A mobile station in packet transfer mode or in MAC-Shared state may receive neighbouring cell system information for GSM neighbouring cells on PACCH. System Information messages are not distributed for 3G neighbouring cells or E-UTRAN neighbouring cells. The neighbouring cell system information is contained in one or more instances of the PACKET NEIGHBOUR CELL DATA message and the mobile station is addressed by its TFI as follows:

– If a PBCCH is allocated in the neighbouring cell, the instances of the message may contain the PSI1, a consistent set of PSI2 and the PSI14 messages;

– If no PBCCH is allocated in the neighbouring cell, the instances of the message may contain the SI3, SI13 and, if available, SI1 messages. If SI1 is broadcast in the target cell, the network shall include the SI1 message as the first SI message contained in the set of PACKET NEIGHBOUR CELL DATA messages, starting from the message with CONTAINER_INDEX=0.

A mobile station, which receives this information shall, independent of NC mode or CCN mode, store the last received set of the information for at least one cell. The received system information can then be used for initial access when entering the designated neighbour cell.

All instances of the PACKET NEIGHBOUR CELL DATA message form a complete container for a certain neighbour cell. The container is addressed by a container identity (CONTAINER_ID) in each instance and optionally by the ARFCN for BCCH and the BSIC of the neighbour cell. The CONTAINER_ID shall then be included in the PACKET CELL CHANGE CONTINUE or the PACKET CELL CHANGE ORDER message together with the ARFCN and the BSIC. This is in order to map the cell identity to the container identity for which neighbour cell information was received in the PACKET NEIGHBOUR CELL DATA messages. If the ARFCN and BSIC are given for a set of PACKET NEIGHBOUR CELL DATA messages, it is sufficient to include this information in only one instance in the set.

In order to ensure a consistent distribution and decoding of (P)SI messages contained in PACKET NEIGHBOUR CELL DATA messages, the following rules shall apply for PACKET NEIGHBOUR CELL DATA messages with the same container identity:

– Whenever the network starts sending a set of PACKET NEIGHBOUR CELL DATA message instances, the first PACKET NEIGHBOUR CELL DATA message instance shall be started with CONTAINER_INDEX=0;

– All subsequent instances of a PACKET NEIGHBOUR CELL DATA message set shall be sent in ascending order of CONTAINER_INDEX value. For retransmission purposes it is allowed to send a PACKET NEIGHBOUR CELL DATA message with the same CONTAINER_INDEX value and the same content more than once;

– Whenever the MS receives a PACKET NEIGHBOUR CELL DATA message instance with CONTAINER_INDEX=0 or with a CONTAINER_INDEX value that is less than the CONTAINER_INDEX value of the last received PACKET NEIGHBOUR CELL DATA message instance, it shall delete any PACKET NEIGHBOUR CELL DATA message instances it may have stored and the extracted system information of the neighbour cell;

– If the MS receives a PACKET NEIGHBOUR CELL DATA message with a different ARFCN and BSIC than was indicated in one or more already received PACKET NEIGHBOUR CELL DATA message instances, it shall delete any PACKET NEIGHBOUR CELL DATA message instances it may have stored and the extracted system information of the neighbour cell;

– 30 s after the reception of the latest PACKET NEIGHBOUR CELL DATA message instance, the MS shall delete any PACKET NEIGHBOUR CELL DATA message instances it may have stored the extracted system information of the neighbour cell.

When the mobile station receives the PACKET CELL CHANGE ORDER or the PACKET CELL CHANGE CONTINUE message the mobile station shall transmit a PACKET CONTROL ACKNOWLEDGMENT message in the specified uplink radio block if a valid RRBP field is received as part of the message; the mobile station may then switch to a new cell. If the mobile station has collected all required instances of the PACKET NEIGHBOUR CELL DATA message for the new cell already when in the old cell, then it may perform access depending on whether the PACKET PSI STATUS (or PACKET SI STATUS if PBCCH is not supported in the new cell) procedures are supported by the network in the new cell (see below). The required instances of the Packet Neighbour Cell Data message include PSI1, a consistent set of PSI2 messages and PSI14 (if the new cell has a PBCCH allocated) or SI3, SI13 and, if available, SI1 messages (if the new cell does not have a PBCCH allocated). If the MS is able to decode the first SI message contained in the set of PACKET NEIGHBOUR CELL DATA messages but it was not the SI1 message, the MS shall conclude that SI1 is not broadcast in that particular cell in determining when packet access is allowed in the cell (see sub-clause 5.5.1.3).

If not all required instances of the PACKET NEIGHBOUR CELL DATA message have been received before the cell change, the MS shall first obtain the PBCCH description (if available) and the missing system information messages before making initial access in the new cell. However, it may switch to the new cell as soon as PSI1 has been received (if PBCCH is supported in the new cell) or SI13 has been received (if PBCCH is not supported in the new cell).

Once all the required system information messages have been received, and if the new cell supports the PACKET PSI STATUS (respectively PACKET SI STATUS) procedures, the mobile station may perform access in the new cell and shall then use these procedures for acquisition of PSI (respectively SI) messages (see sub-clause 5.5.1.4.3). If the PACKET PSI STATUS (respectively PACKET SI STATUS) procedures are not supported by the network in the new cell, then the MS is still required to make at least one attempt to receive the complete set of PSI messages on PBCCH (respectively make at least one attempt to receive other SI messages that may be scheduled within one TC cycle on BCCH) prior to perform access in the new cell (see sub-clauses 5.5.1.2 and 5.5.1.3).

8.8.2 CCN setting procedure

The network uses the parameter CCN_ACTIVE in the GPRS Cell Options IE on the BCCH (SI13) or PBCCH (PSI1/PSI13/PSI14) to indicate in the cell whether CCN is enabled for cell reselection towards GSM cells.

The network uses the parameter 3G_CCN_ACTIVE on the BCCH (SI2quater) or PBCCH (PSI3quater) to indicate in the cell whether CCN is enabled for cell reselection towards 3G cells (including UTRAN CSG cells).

The network uses the parameter E-UTRAN_CCN_ACTIVE on the BCCH (SI2quater) to indicate in the cell whether CCN is enabled for cell reselection towards E-UTRAN cells (including E-UTRAN CSG cells).

– If CCN_ACTIVE is not provided or it indicates that CCN is disabled in the cell, the mobile stations shall not follow the CCN procedures towards GSM cells. CCN_ACTIVE can also be individually sent to the mobile station in either a PACKET MEASUREMENT ORDER, a PACKET CELL CHANGE ORDER message or a PS HANDOVER COMMAND message. In the latter cases, the setting applies in the target cell;

– If 3G_CCN_ACTIVE is not provided or it indicates that CCN is disabled in the cell, the mobile stations shall not follow the CCN procedures towards 3G cells. 3G_CCN_ACTIVE can also be individually sent to the mobile station in either a PACKET MEASUREMENT ORDER, a PACKET CELL CHANGE ORDER message or a PS HANDOVER COMMAND. In the latter cases, the setting applies in the target cell;

– If E-UTRAN_CCN_ACTIVE is not provided or it indicates that CCN is disabled in the cell, the mobile station shall not follow the CCN procedures towards E-UTRAN cells. E-UTRAN_CCN_ACTIVE can also be individually sent to the mobile station in either a PACKET MEASUREMENT ORDER, a PACKET CELL CHANGE ORDER message or a PS HANDOVER COMMAND. In the latter cases, the setting applies in the target cell;

– If CCN_ACTIVE indicates that CCN is enabled in the cell and a mobile station determines a cell re-selection towards one of its neighbouring GSM cells is required, it shall first check the corresponding CCN_SUPPORTED parameter if available. This allows the network to enable CCN mode towards some but not all neighbour GSM cells.

NOTE: It is not possible for the network to enable CCN mode towards individual 3G or E-UTRAN neighbour cells.

An individual setting of CCN shall take precedence over the broadcast setting of CCN. The latest individual ordered setting of CCN is the valid one. An individual setting of CCN is only valid within the cell it is ordered for. CCN is applicable to a mobile station in Network Control mode NC0 and NC1 but not in mode NC2 that takes precedence over CCN.

8.8.2a CCN support description

The CCN Support description contains the CCN_SUPPORTED parameter for each GSM cell of the GSM Neighbour Cell list.

If there is no PBCCH allocated in the cell, see 3GPP TS 44.018.

If there is a PBCCH allocated in the cell, the parameter can be provided in the PSI3 message or any instance of the PSI3bis message. In that case, the bitmap applies to the GSM Neighbour Cell list having the same PSI3_CHANGE_MARK as the message in which it is provided.

CCN_SUPPORTED parameter can also be provided in a PACKET CELL CHANGE ORDER or a PACKET MEASUREMENT ORDER message. In this case, the bitmap applies to the updated GSM Neighbour Cell list.

Each CCN_SUPPORTED bit of this description relates to indices of the GSM Neighbour Cell list, starting with index 0. The CCN Support description may be received before the corresponding GSM Neighbour Cell list.

Indices exceeding the value 95 or the number of cells in the GSM Neighbour Cell list (whichever is the lowest) shall be ignored. If there are fewer indices than the number of cells in the GSM Neighbour Cell list, the value 0 shall be assumed for the missing bits.

When this information is not present but CCN is enabled in the serving cell, the mobile station shall assume that CCN is enabled towards all neighbour cells.

8.8.3 Cell Change Notification procedure

If CCN is enabled towards the target cell (see sub-clause 5.5.1.1a), the mobile station shall behave as in network control mode NC0 or NC1 up to the point when a new cell has been chosen. If the target cell is a GSM cell, the mobile station shall then check the CCN_SUPPORTED parameter, if available, that was last received for that cell. This parameter can be sent on BCCH or PBCCH or individually in PACKET MEASUREMENT ORDER or PACKET CELL CHANGE ORDER or PS HANDOVER COMMAND messages.

If for a GSM cell the CCN_SUPPORTED parameter is available and if it indicates that CCN mode shall not be entered towards that cell, then the mobile station shall perform the cell change and not enter CCN mode. If the cell reselection is triggered by the path loss criterion parameter C1 becoming negative, the mobile station may perform the cell change without entering the CCN mode.

If the target cell is a GSM cell and the CCN_SUPPORTED parameter is available and if it indicates that CCN mode shall be entered towards that cell or if the CCN_SUPPORTED parameter is not available, then instead of performing the cell change, the mobile station shall start timer T3206 and enter the CCN mode. At the first possible opportunity, the MS shall then, when in CCN mode, inform the network about the proposed target cell by sending a PACKET CELL CHANGE NOTIFICATION message, stop timer T3206, start timers T3208 and T3210.

If the target cell is a 3G cell and if CCN is activated towards 3G cells or if the target cell is an E-UTRAN cell and the mobile station supports "CCN towards E-UTRAN, E-UTRAN Neighbour Cell measurement reporting and Network controlled cell reselection to E-UTRAN" and if CCN is activated towards E-UTRAN cells, then instead of performing the cell change, the mobile station shall start timer T3206 and enter the CCN mode. At the first possible opportunity, the MS shall then, when in CCN mode, inform the network about the proposed target cell by sending a PACKET CELL CHANGE NOTIFICATION message, stop timer T3206, start timers T3208 and T3210.

If the target cell is a GAN cell and the CCN_SUPPORTED parameter is available and if it indicates that CCN mode shall be entered towards that cell or if the CCN_SUPPORTED parameter is not available, then instead of performing the cell change, the mobile station shall start timer T3206 and enter the CCN mode. At the first possible opportunity, a mobile station may, when in CCN mode, send a Packet Cell Change Notification message that includes the ARFCN/BSIC for the GAN cell and indicates an RXLEV of 63 for the GAN cell and start timers T3208 and T3210.

The PACKET CELL CHANGE NOTIFICATION message shall contain the identity of the proposed target cell. A GSM cell shall be identified by the ARFCN for the BCCH and the BSIC. A 3G-FDD cell shall be identified by FDD_ARFCN, Bandwidth_FDD and Scrambling Code. A 3G-TDD cell shall be identified by TDD_ARFCN, Bandwidth_TDD, Cell parameter and Sync Case. An E-UTRAN cell shall be identified by EARFCN, Measurement Bandwidth and Physical Layer Cell Identity.

The PACKET CELL CHANGE NOTIFICATION message shall also contain measurement reports for the proposed cell and for six other (see 3GPP TS 45.008) neighbour cells, if available. If no 3G neighbour cell is available for reporting, and the 3G CCN Measurement Report Struct is included in the message, the MS shall set the N_3G field to 0 and the 3G_CELL_LIST_INDEX field to 127, see sub-clause 11.2.3a. If 3G Neighbour cells are reported, the type of report for FDD cells are specified by the FDD_REP_QUANT parameter and the number of 3G reports in the message are specified by the parameters FDD_MULTIRAT_REPORTING and TDD_MULTIRAT_REPORTING. These parameters may either be broadcast on BCCH or PBCCH or sent to the mobile station in a PACKET MEASUREMENT ORDER or PACKET CELL CHANGE ORDER message. If E-UTRAN Neighbour cells are reported, the type of report is specified by the E-UTRAN_REP_QUANT parameter and the maximum number of E-UTRAN reports in the message is specified by the parameter E-UTRAN_MULTIRAT_REPORTING. These parameters may either be broadcast on BCCH or sent to the mobile station in a PACKET MEASUREMENT ORDER message.

When a multi-RAT MS attempts cell change to a UTRAN (respectively E-UTRAN) CSG cell or a detected hybrid cell if the cell’s CSG ID is included in the CSG Whitelist of the MS and CCN procedures are used, if the CSG Cells Reporting Description IE has been received from the network and the mobile station supports CSG Cells Reporting, the mobile station shall include in the PACKET CELL CHANGE NOTIFICATION message the CSG-ID and the routing parameters of the target cell. In this case, if a cell change failure occurs, the subsequent transmission of a PACKET CELL CHANGE FAILURE message shall contain the routing parameters of the target cell.

In CCN mode the mobile station shall continue the data transfer and store neighbour cell system information if received in instances of the PACKET NEIGHBOUR CELL DATA message, but not perform the cell change. At receipt of the first PACKET NEIGHBOUR CELL DATA message or PACKET CELL CHANGE CONTINUE message or PACKET CELL CHANGE ORDER message or PS HANDOVER COMMAND message, the mobile station shall stop the timer T3210.

The mobile station shall retransmit the PACKET CELL CHANGE NOTIFICATION message once at the first possible opportunity when the timer T3210 expires.

The mobile station shall leave CCN mode when either CCN is no longer enabled (towards all GSM, 3G or E-UTRAN neighbour cells with the CCN_ACTIVE/3G_CCN_ACTIVE/E-UTRAN_CCN_ACTIVE bit or towards the cell that had been re-selected) or when the network has responded with a PACKET CELL CHANGE CONTINUE or PACKET CELL CHANGE ORDER message or a PS HANDOVER COMMAND message or when either of the timers T3206 or T3208 have expired.

If the mobile station has been individually ordered to enable CCN, the order is only valid within the cell where the order is given. When a cell change has been performed using the cell reselection procedure, the mobile station shall use CCN in the new cell only if individually ordered in the previous cell with the PACKET CELL CHANGE ORDER message or if individually ordered or broadcast in the new cell. When a cell change has been performed using the PS handover procedure, the mobile station shall enable CCN in the new cell only if individually ordered in the previous cell with the PS HANDOVER COMMAND message or if individually ordered or broadcast in the new cell.

Upon receiving a PACKET CELL CHANGE NOTIFICATION message, the network shall ignore any 3G neighbour cell report with a 3G_CELL_LIST_INDEX equal to 127.

After receiving a PACKET CELL CHANGE NOTIFICATION message from the mobile station the network can behave in different ways as described below:

1) The network responds with a PACKET CELL CHANGE CONTINUE message.
If a mobile station as response to a PACKET CELL CHANGE NOTIFICATION message receives a PACKET CELL CHANGE CONTINUE message without receiving any neighbour cell system information, the mobile station shall stop timer T3208, stop timer T3210 if still running, leave CCN mode and continue cell reselection in NC0/NC1 mode.

2) The network sends first necessary system information for the cell proposed in the PACKET CELL CHANGE NOTIFICATION message if the proposed target cell is a GSM cell in one or more instances of the PACKET NEIGHBOUR CELL DATA message and sends then a PACKET CELL CHANGE CONTINUE message.
The mobile station shall store the received system information as specified in sub-clause 8.8.1. When the first instance of the PACKET NEIGHBOUR CELL DATA message is received, the mobile station shall stop timer T3210 if still running. When the PACKET CELL CHANGE CONTINUE message is received, the mobile station shall stop timer T3208, leave CCN mode and continue the cell reselection in NC0/NC1 mode irrespective of the cell indicated in the ARFCN and BSIC parameters in the PACKET CELL CHANGE CONTINUE message.

3) The network sends first necessary system information for the cell proposed in the PACKET CELL CHANGE NOTIFICATION message if the proposed target cell is a GSM cell, or for any other GSM cell, in one or more instances of the PACKET NEIGHBOUR CELL DATA message and sends then a PACKET CELL CHANGE ORDER message or a PS HANDOVER COMMAND message.

The mobile station shall store the received system information as specified in sub-clause 8.8.1. When the first instance of the PACKET NEIGHBOUR CELL DATA message is received, the mobile station shall stop timer T3210 if still running. When the PACKET CELL CHANGE ORDER message is received, the mobile station shall stop timer T3208, leave CCN mode and follow the procedures as specified for the PACKET CELL CHANGE ORDER message (sub-clause 8.4) and in sub-clause 8.8.1. When the PS HANDOVER COMMAND message is received, the mobile station shall stop timer T3208, leave CCN mode and follow the procedures as specified for the PS HANDOVER COMMAND message in sub-clause 8.10.4.

4) The network orders the mobile station into NC2 mode.
A mobile station may in response to a PACKET CELL CHANGE NOTIFICATION message receive a PACKET MEASUREMENT ORDER message from the network indicating NC2 mode. When the mobile station receives the NC2 order it shall leave CCN mode, stop timer T3208, stop timer T3210 if still running, and go into NC2 mode.

When the NC2 mode has been ordered, the network may send PACKET NEIGHBOUR CELL DATA messages on the PACCH before sending the PACKET CELL CHANGE ORDER message to the mobile station. When the NC2 mode has been ordered, the network shall send PACKET NEIGHBOUR CELL DATA messages on the PACCH before sending the PS HANDOVER COMMAND message (see sub-clause 8.10.2) to the mobile station except if these messages are not needed by the mobile station (i.e. for the case of PS handover to a GAN cell).

5) No network response
When timer T3210 expires, the mobile station shall retransmit once the PACKET CELL CHANGE NOTIFICATION message at the first possible opportunity.
When timer T3208 expires, the mobile station shall leave CCN mode and continue cell reselection in NC0/NC1 mode as described in sub-clause 5.5.1.1 and in 3GPP TS 45.008.

The CCN mode is only valid in packet transfer mode or in MAC-Shared state. If the mobile station is in CCN mode when entering packet idle mode or MAC-Idle state, the mobile station shall stop the timers T3206 and T3208, stop timer T3210 if still running, leave CCN mode and continue the cell reselection procedure according to the NC0/NC1 procedures. If PACKET NEIGHBOUR CELL DATA messages are received on the PACCH before entering packet idle mode or MAC-Idle state and the cell identity parameters are included, this information may then be used at the next cell change.

If the cell reselection criteria have changed during the time the MS is in CCN mode but the path loss criterion parameter C1 remains positive, the MS shall, without notifying the network about the new preferred cell, remain in CCN mode until the criteria for CCN mode are no longer fulfilled. When leaving CCN mode the MS shall obey the new criteria according to the normal rules as specified in sub-clause 5.5.1.1 and in 3GPP TS 45.008 unless a PACKET CELL CHANGE ORDER or a PS HANDOVER COMMAND message has been received (see bullet 3 above). If the path loss criterion parameter C1 becomes negative while the MS is in CCN mode, the MS may leave the CCN mode without notifying the network and perform the cell change.